The present invention generally relates to printers, and more particularly to a serial type electrophotography printer provided with a fixing unit within a carriage which carries out the printing.
Recently, a serial type electrophotography printer has been developed to satisfy the demands to reduce both the cost and size of electrophotography type recording apparatuses. The serial type electrophotography printer is provided with an electrophotography process means on a carriage which carries out the printing. According to such a serial type electrophotography printer in general, a transfer unit transfers an image on a recording sheet which is transported, by moving the carriage in a direction perpendicular to a transport direction of the recording sheet, and the image is fixed by a fixing unit which has the form of a roller and is arranged on the downstream side of the transfer unit along the transport direction. In order to further reduce the size of the printer, a printer provided with the fixing unit within the carriage has also been developed. In such a printer, there are demands to print an image having a high quality and to guarantee safety.
FIGS. 1A through 1C are diagrams showing important parts of a conceivable serial type electrophotography printer. FIG. 1A shows a plan view of a carriage part, FIG. 1B shows a cross section taken along a line A--A in FIG. 1A, and FIG. 1C shows a cross section taken along a line B--B in FIG. 1A.
In FIGS. 1A through 1C, a carriage 11 is made up of a process part 12 and a fixing unit 13. The carriage 11 is guided by shafts 14a and 14b, and is moved in a main scan direction by a driving motor (not shown) above a transfer unit (printing platen) 15. This main scan direction is perpendicular to a transport direction of a recording sheet 17. Transport rollers 16a and 16b are arranged on both sides of the carriage 11 along the transport direction. The recording sheet 17 is transported by the transport rollers 16a and 16b between the transfer unit 15 and the carriage 11.
An image bearing member 21 is provided on the process part 12 of the carriage 11, and this image bearing member 21 rotates at a peripheral speed synchronized to the movement of the carriage 11. The surface of the image bearing member 21 is uniformly charged by a charger 22, and an electrostatic latent image is formed on the surface of the image bearing member 21 by an exposure unit 23. The electrostatic latent image is visualized into a toner image by a developing roller 24. The toner image formed on the image bearing member 21 is transferred onto the recording sheet 17 by the transfer unit 15 which confronts the image bearing member 21 with the recording sheet 17 interposed therebetween. The residual toner on the image bearing member 21 is removed by a cleaner 25.
The surface of the cleaned image bearing member 21 is charged again by the charger 22, and a printing process is repeated similarly as described above. When the printing amounting to a predetermined width ends, the recording sheet 17 is transported a predetermined amount by the transport rollers 16a and 16b, and the carriage 11 is returned to a predetermined position (home position) so as to carry out a printing operation again.
The transfer of the toner image by the transfer unit 15 is made by applying a predetermined voltage across the transfer unit and the image bearing member 21. Hence, the transfer unit includes a heat resistant conductor portion 15b which is made of a conductive rubber or the like and is formed on a substrate 15a.
On the other hand, the fixing unit 13 is provided with a fixing roller 26 and a heater part 27 arranged in a vicinity of the fixing roller 26. The heater part 27 preheats the fixing roller 26 to a predetermined temperature before the printing operation of the process part 12 starts. The temperature of the fixing roller 26 during the printing is controlled by detecting the temperature by a temperature detector (not shown) such as a thermistor. The fixing unit 13 moves together with the process part 12 and carries out the fixing process immediately after the transfer process of the process part 12.
FIG. 2 shows a cross section of another conceivable carriage. In FIG. 2, those parts which are the same as those corresponding parts in FIGS. 1A through 1C are designated by the same reference numerals, and a description thereof will be omitted.
In the carriage 11 shown in FIG. 2, a halogen lamp (infrared ray lamp) 28 is provided in the fixing unit 13 as a heater means, and in addition, a reflecting mirror 29 is arranged around the periphery of this halogen lamp 28. Hence, this fixing unit 13 uses direct irradiation of the halogen lamp 28 when fixing the toner image that is transferred onto the recording sheet 17.
However, when the carriage 11 returns to the home position or is in a standby state, the developing roller 24, the cleaner 25 and the charger 22 are pushed against the image bearing member 21 for a long time. As a result, there are problems in that the material components of the developing roller 24 and the like adhere on the image bearing member 21, and unwanted toner adheres on the image bearing member 21 to cause undesirable effects at the time of the printing such as developing a residual image. In addition, there is a problem in that it is difficult to carry out a charging operation to restore the potential of the image bearing member 21.
On the other hand, in the case of the so-called cleanerless process which eliminates the cleaner part so as to reduce the size of the cartridge and to improve the handling by the user, it is necessary to recover the residual toner on the image bearing member 21 by the developing roller 24 after the transfer process. In other words, unless the surface of the image bearing member 21 is completely cleaned by this recovery operation (developing operation) of the developing roller 24 before starting the printing from the home position, there is a problem in that the residual toner on the surface of the image bearing member 21 will be transferred onto the recording sheet 17 during a next printing operation, thereby deteriorating the quality of the printed image.
Moreover, when the fixing roller 26 is heated by the heater part 27 in a vicinity thereof in the fixing unit 13, the temperature difference on the surface of the fixing roller 26 becomes large when the fixing roller 26 is stationary. In this case, it is difficult to quickly start the printing operation because of the large temperature difference. Further, in order to heat a low temperature part of the fixing roller 26 to a predetermined temperature, it is necessary to set the heating temperature greater than a rated value. But in this case, there are problems in that a short-circuit may occur due to the melting of the heated part, and a member such as a flange which is arranged in a vicinity of the fixing roller 26 and holds the fixing roller 26 may also melt.
Moreover, when the fixing roller 26 remains in the stationary state for a long time, it becomes necessary to provide a means for dispersing the toner with respect to the fixing roller 26. Consequently, there is a problem in that it is difficult to reduce the size of the printer while enabling easy maintenance.
In addition, there is also a problem in that the toner which is not fixed may adhere on the fixing roller 26 during the printing operation, and this toner may be transferred to the recording sheet 17 during the next printing operation to thereby deteriorate the quality of the printed image.